large kiln firing

[ironman55]

I was wondering if a 23X35 piece is too big to fire without problems? Does the glass have to be thicker the bigger it is? I know that you have to slow down the heat and anneal process the thicker it is. Can you still go from lets say 1650 to 960 at the 9999 rate and then say 60 min hold and 80 ph to 800 and hold 90 sound too fast? Anyway still learning. Thanks in advance

[DonMcClennen]

I suggest your 9999 drop from fuse temp should be stopped at 1000F not 950F... you then move down slower as you are entering the anneal zone (80F dph to 950F) hold for anneal.. then 80F dph down to 700F then 125F dph to room temp before opening.

[Bert Weiss]

Until you get down to the 1" pendant size scale, what matters is thickness. I can put a piece of 10mm thick float glass 40" x 84" in my kiln and heat it up at 600ºF per hour and anneal it exactly the same as I would a 4" coaster, of the same thickness.

Not exactly all that matters, you also have to be able to heat the glass evenly enough to prevent thermal shock, and to anneal. In order to anneal the glass you have to be able to get the entire mass of the glass within 5ºC for 15 seconds. So, if you have your glass too close to the kiln walls, this might be an impossible task.

[Morganica]

The point of the anneal, really, is to allow heat to dissipate from the glass evenly, so what you're trying to do is ensure that all parts of the glass stay at roughly the same temperature all the way down. If the glass is uniformly thin, no matter how wide and long it is, the heat is still able to escape out the top surface at roughly the same rate and you're OK.

Where it (mainly) gets complicated is when you've got uneven thicknesses, angles in the glass that might be releasing heat differently, or when you've got nearby heat sinks radiating heat back into one part of the glass and not another (such as when the class is very close to the heat-absorbing walls of the kiln). All those things screw up the steady heat dissipation you need for proper annealing, so you'll either slow down the cooling schedule to compensate, baffle (insulate) the cooler parts of the glass...or admit that this particular project won't work in your kiln.

There can be a second complication: Since the underside of the glass (next to the kilnshelf) doesn't cool (and therefore contract) as fast as the top surface, the piece can warp up during cooling. It's not usually noticeable unless the piece is relatively long and thin, but if you've got a narrow rectangle you can sometimes get a definite upward bow and sometimes even enough induced stress to crack the piece.

You can alleviate it by warming the glass up again slightly, after it's stopping moving, just enough to relax it back down to the shelf and eliminate the stress. Then you go through the rest of the anneal as usual.

[Hedda]

You can alleviate it by warming the glass up again slightly, after it's stopping moving, just enough to relax it back down to the shelf and eliminate the stress. Then you go through the rest of the anneal as usual.

Cynthia, could you please elaborate a little on this important adjustment that you are mentioning? In the past i had the very upsetting problem of upward arching you describe with long and narrow pieces and now i have two similar pieces , about 20x60 cm in the kiln, just waiting to be fired. Maybe that they were waiting for this discussion, too
Could help me a lot to compare your firing times of a normal piece with the times for a similar piece, just long and narrow. Do you think I could aleviate the arching problem also by ramping up at a much slower rate, like half the rate for a piece of "normal" dimensions?
Thank you
Hedda

[Bert Weiss]

Only once did I experience a big sheet of glass warping. I discovered a fairly large gap between bricks that are just placed around my perimeter. I heated the glass back to anneal soak, re-annealed, and then it un-warped.

I have seen pictures of people firing glass very close to kiln walls. I would never try this. I hope it works for them.

Glass blowers can anneal quite a bit faster than fusers. This is because only a small portion of their glass is sitting against a kiln shelf. Therefore the glass can stabilize at the air temperature faster.

[Dick]

You really can not answer this question, without some more info. Especially thickness but is this in a mold, covered with investment, and if so what kind? What type and color of glass. All of this comes into play. It is correct that you may want to hold at 1000 and then go to annealing or just a longer annealing. Then the more critical part comes into play as to how slowly you cool.